You could setup an order added rule so when you add shrimp tacos it will add another free
Create an add order action and input shrimp tacos as the order to be added and set the price to zero. Put that action in an order added rule and add constraints for the days you want it to work and for shrimp tacos so it will only fire if that item is added
You could customise it further with an ask question to, so when shrimp tacos is added it asks the cashier if the customer wants another free, you then press yes to add the free one or cancel to not add the free one (some people may not want another taco even if its free so it saves manually removing the item after its already been added)
another option is to look at the gift rule, and use those action combined with adding your taco to automatically gift another taco when one is added. Benefit of this is that it will show as GIFT on the ticket and receipt